A stepper is a device utilized in fabrication of integrated circuits. Steppers play an essential role in a photolithographic process where microscopic and nano-sized circuit elements are created on wafers.
In the photolithographic process, elements of the circuit to be created on the wafer are reproduced in a pattern of transparent and opaque areas formed on a surface of a photomask. The stepper passes light through the mask to form an image of the mask pattern on the wafer. Usually, the image is focused and reduced by a lens of the stepper. The image is then projected onto the surface of the wafer which is coated with a photoresist.
After exposure in the stepper, the coated wafer is developed in the manner of photographic film, causing portions of the photoresist to dissolve in certain areas in accordance to the exposure dose in the areas receiving light. The areas of the photoresist and no photoresist reproduce a pattern on the mask. The developed wafer is then exposed to an etching process as is known to those skilled in the art.
Photo-masks which are used in the steppers play a large role as to the quality of the images which are to be printed on the wafers.
Referring to FIG. 1, a conventional stepper 10 includes a wafer stage 12 with a wafer 14 attached thereto, an illumination system 16, a mask 18, and an optical system 20 which includes a condenser lens 22 between the light source and the mask, and a projection (reduction) lens 24 between the mask and the wafer.
As it is seen in FIGS. 1 and 2, the conventional stepper 10 uses a conventional mask 18 which has a continuous clear region 26 formed in an opaque mask plate 28. The clear region 26 of the conventional mask is formed at a location, and sized as well as shaped to permit “imprinting” of a micro- or nano-feature on a substrate (wafer) as is conventional in photolithography. In order to “write” a pattern, e.g. the continuous clear region 26, on the mask 18, a focused ion beam or electron beam (not shown) is scanned over the opaque mask plate addressing all pixels corresponding to the clear region. In this mask, the fill-factor, e.g. the ratio of the clear-to-opaque area may be quite large. The necessity to expose all pixels of the opaque mask plate corresponding to the clear region requires a lengthy writing process for the conventional mask. Therefore, it would be highly desirable to reduce the “writing time” in the mask fabrication.
The interaction between light and a hole in an opaque screen has been the object of curiosity in technology application for centuries. Grimaldi (F. M. Grimaldi, Physico-Mathesis De Lumine, Coloribus, et Iride, 9, 1665) first described diffraction from a circular aperture thereby providing an experimental basis for classical wave optics in the 17th century. Conventional diffraction theory of light transmission through a sub-wavelength aperture predicts a strongly attenuated transmitted beam (H. A. Bethe, Phys. Rev. 66, 163, 1944; T. W. Ebbesen, et al., Nature (London) 391, 667, 1998).
However, an interesting transmission phenomenon is seen to take place when light interacts with an array of sub-wavelength apertures in an opaque metal sheet. In 1998, Ebbesen, et al. made the observation of transmission efficiency from sub-wavelength circular apertures which was orders of magnitude greater than predicted by a standard aperture theory. Experiments provided evidence that the unusual optical property was due to the coupling of light with plasmons on the surface of the periodically patterned metal film. It was also observed that arrays of such holes display highly unusual zero-order transmission spectra at wavelengths larger than the array period beyond which no diffraction occurs. In addition, sharp peaks in transmission were observed at wavelengths as large as 10 times the diameter of the cylindrical apertures.
It is believed that light incident on a metal thin film establishes oscillations in the mobile charge density (ripples in the “Fermi sea”). These ripples, or plasmon excitations in the metal foil give rise to an evanescent mode of re-radiation that has been used in the past for contact printing. In addition, the ripples also excite the cavity modes of circular apertures in the thin film. These cavity modes act as intense light sources propagating into the far-field, drawing energy from their surroundings on which light is incident. The net transmission is far greater than the aperture area would dictate if taken alone.
It would be highly desirable to apply the plasmonic effect and extraordinary transmission phenomenon of the light interaction with an array of sub-wavelength holes formed in an opaque metal sheet to provide inexpensive stepper for an ultra-high resolution sub-wavelength lithographic system for fabrication of semiconductor integrated circuits, data storage, as well as in microscopy, bio-photonics, etc.